Sensor integrated slit for pushbroom hyperspectral system

ABSTRACT

An entry slit panel for a push-broom hyperspectral camera is formed at least partly from a silicon wafer on which at least one companion sensor is fabricated, whereby the companion sensor is co-planar with the slit and detects light imaged on the panel but not on the slit. In embodiments, the companion sensor is a panchromatic sensor or a sensor that detects light outside the wavelength range of the camera. At least a region of the wafer is back-thinned to a thickness appropriate for a diffraction slit. The slit can be etched or laser cut through the thinned region, or formed between the wafer and another wafer or a conventional blade. The wafer can be back-coated or metalized to ensure its opacity across the camera&#39;s wavelength range. The companion sensor can be located relative to the slit to detect scene features immediately before or after the hyperspectral camera.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/486,793, filed May 17, 2011, herein incorporated by reference in itsentirety for all purposes.

FIELD OF THE INVENTION

The invention relates to camera systems, and more particularly, topushbroom hyperspectral cameras.

BACKGROUND OF THE INVENTION

Hyperspectral imaging is essentially a three-dimensional imaging method,whereby the two spatial dimensions of an image are augmented by a thirddimension in which the wavelength spectrum of each pixel is encoded. Theresult is a hyperspectral “data cube” of information, which can behighly useful in a wide range of application, including mining andgeology (e.g. looking for ores and petroleum deposits), agriculture, andmilitary surveillance.

A hyperspectral camera collects data as a set of images, where eachimage represents a wavelength range or band of the electromagneticspectrum, where the detected wavelengths need not be limited to thevisible spectrum. Depending on the wavelength range of each image, themethod may be referred to as “ultraspectral” (for very fine spectralresolution), “hyperspectral” (for intermediate spectral resolution), or“multispectral” for fewer and/or broader spectral bands that span a widerange of wavelengths. All such related methods are referred togenerically herein as “hyperspectral” unless otherwise required by thecontext.

Because each image produced by a hyperspectral camera includes only thelight from a single wavelength band, in contrast with a “panchromatic”image that combines light over a wide range of wavelengths, ahyperspectral image will generally have a lower resolution than apanchromatic image obtained under similar conditions.

There are two basic types of camera for obtaining hyperspectral data. A“staring array” hyperspectral camera repeatedly images an entire scenethrough a tunable filter while advancing the wavelength of the filter soas to obtain the images for the different wavelength bands. Oneadvantage of this approach is that the detected wavelength bands neednot be contiguous, but can be selected according to the wavelength bandsof interest for a particular application. Nevertheless, while staringarray hyperspectral cameras are useful for some applications, thelimited tuning rate of the optical filter can limit the data throughput.

The other basic type of hyperspectral camera is a “push-broom”hyperspectral camera. In contrast to a staring array camera, whichimages an entire scene at once and then steps or sweeps through a seriesof wavelength bands to obtain the data cube, a push-broom hyperspectralcamera obtains only one line of an image at a time, but simultaneouslyobtains the full spectral information for each point on the line. Theimaging line is then stepped or swept through the image to provide thecomplete data cube. Push-broom imaging is especially useful formonitoring industrial processes, where items are typically moving pastthe camera at a constant speed. Similarly, push-broom imaging is wellsuited to airborne applications, where the camera is steadily movedacross a scene. In such applications, the push-broom approach ofmeasuring the full wavelength information of a line at the same instantensures that all the wavelength information is truly measured from thesame portion of the scene, even though the scene is moving during themeasurement.

Push-broom cameras obtain wavelength spectrum information by focusing animage of a scene onto a line-selecting slit, and then either reflectingthe light that passes through the slit from a reflective grating, or bypassing the light through a transmission grating or through adiffraction slit. The latter case is an especially simple and effectiveapproach, because a single slit can serve as both the line-selectingslit and the diffraction slit.

The push-broom design avoids the delays associated with filter tuning ina staring array camera. However, a push-broom camera cannot selectspecific wavelength bands, but can only provide spectral informationover a continuous range of wavelengths.

FIG. 1A is a perspective view illustrating the basic components of asingle-slit push-broom hyperspectral camera of the prior art. A lens 100focuses an image 102 onto image panel 104 that is penetrated by a thinand narrow diffraction slit 106. Typically, the diffraction slit 106 iswide enough to span most of the image 102. For example, typicaldimensions might be a 25 mm diameter circular image 102 focused onto a20 mm wide slit 106 that is 20 microns high.

Light 108 that passes through the slit 108 falls on a hyperspectraldetector array 110 mounted on a detector panel 112. The data collectedby the detector array 110 represents spatial information along one axis114, and wavelength information along the other axis 116. Repetition ofthis measurement during movement 118 of the scene past the lens, ormovement 118 of the camera past the scene, provides information in theother spatial dimension, and allows collection of the complete datacube.

FIG. 1B is a detailed front view of the image 102 focused on the slit108 of FIG. 1A. Note that the height of the slit has been exaggerated tomake it perceptible in the drawing. It is evident that most of the image118 focused by the lens 100 does not pass through the slit 106 and isnot used.

It is sometimes desirable to compensate for the shortcoming of ahyperspectral camera by simultaneously gathering data from the sceneusing other types of detectors as “companion” sensors. For example, thelower resolution of a hyperspectral camera can be compensated bysimultaneously detecting light from the scene using one or morepanchromatic sensors. Also, light can be detected from additionalspectral bands of interest, for example light resulting from “LightDetecting and Ranging” (LIDAR), by using separate sensors that areeither intrinsically sensitive to the wavelength bands of interest orinclude appropriate filters.

However, including companion sensors can require that the light from thelens pass through a splitter and be shared between the hyperspectralsensor and the companion sensors, thereby reducing the light availableto the hyperspectral camera, and reducing the quality of thehyperspectral result. Another possibility is to provide light to thecompanion sensors using a separate optical system. However, thisapproach consumes significantly more space than the hyperspectral cameraby itself. Also, it can be difficult to register the data from thecompanion sensors with the hyperspectral image, due to inevitablemisalignments between the two optical systems.

What is needed, therefore, is an apparatus for detecting push-broomhyperspectral data cube images that includes companion sensors but doesnot reduce the amount of light reaching the hyperspectral sensor anddoes not require separate companion sensor optics.

SUMMARY OF THE INVENTION

An apparatus for detecting push-broom hyperspectral data cube imagesincludes simultaneous detection of light from the scene using companionsensors without reducing the amount of light reaching the hyperspectralsensor and without requiring separate companion sensor optics. Theapparatus takes advantage of the unused portions 118 of the scene imagethat are focused by the lens 100 onto the image panel 104 but do notpass through the diffraction slit 106. The fact that the additional datafrom the companion sensors is acquired through the same lens 100 and atnearly the same time and view angle as the hyperspectral data greatlysimplifies the process of registering the companion sensor data with thehyperspectral image, since the optics are shared and hence inherentlyaligned.

Simply attaching companion sensors to the image panel 104 would beproblematic, because the packaging of the companion sensors wouldconsume space and separate the companion sensors from each other andfrom the diffraction slit 106. Also, the packaging of the companionsensors would tend to locate the companion sensors above the imagingplane of the lens 100.

The apparatus of the present invention overcomes these problems by usingthe silicon die or wafer on which the companion sensors are fabricatedas an integral part of the diffraction slit assembly, either byfabricating the diffraction slit with one or both blades made of acompanion sensor die, or by etching or laser cutting the diffractionslit directly into the silicon die.

At least a region of the silicon die is reduced in width to a thicknessthat is suitable for the diffraction slit by back-thinning it using anyof the techniques that are routinely used to produce back-illuminatedCharge Coupled Device (CCD) sensors. In embodiments, the back surface ofthe die is coated or aluminized to ensure that the thinned silicone isopaque over the full range of wavelengths to be detected by thehyperspectral sensor array.

In embodiments, companion sensors occupy locations where they willobtain data immediately after the hyperspatial sensor. In some of theseembodiments, data obtained by the hyperspectral sensor is used toidentify regions of interest in the scene, and to cue acquisition orspecial processing of data by the companion sensors. In otherembodiments, companion sensors occupy locations where they will obtaindata immediately before the hyperspatial sensor. In some of theseembodiments, data obtained from a companion sensor is used to cueactions by the hyperspatial sensor. For example, data storage capacitycan be conserved by using the companion sensors to cue the activation ofthe hyperspatial sensor, so that hyperspatial data is obtained only forscenes of interest.

One general aspect of the present invention is an image panel for use ina hyperspectral camera that is configured to scan a scene and obtainspectral image data over a defined range of wavelengths. The image panelincludes a silicon die having a planar image surface, a rear surface,and a die thickness, a thinned region of the silicon die having adiffraction thickness that is equal to or less than the die thickness, adiffraction slit formed at least in part by the thinned region of thedie, the diffraction slit having a width and a height, and at least onecompanion sensor fabricated in the silicon die and co-planar with thediffraction slit, the companion sensor being located such that an imageof a scene focused onto the diffraction slit will also be focused ontothe companion sensor.

In embodiments, the thinned region of the die extends to a side of thedie, and the diffraction slit is bounded in part by a segment of thethinned region at the side of the die.

In some embodiments, the diffraction slit penetrates the thinned regionof the die. In other embodiments the companion sensor is a panchromaticsensor.

In various embodiments the companion sensor is configured to detectlight at a wavelength that is outside of the defined range ofwavelengths. In certain embodiments a coating is applied to at least aportion of the rear surface, the coating rendering the portion of therear surface substantially opaque to light over the defined range ofwavelengths.

In some embodiments at least a portion of the rear surface is metalized,the metalized portion of the die being substantially opaque to lightover the defined range of wavelengths. In other embodiments thecompanion sensor is located such that it will receive light from aportion of the scene immediately before light from the portion of thescene passes through the diffraction slit as the scene is scanned.

In certain embodiments the companion sensor is located such that it willreceive light from a portion of the scene immediately after light fromthe portion of the scene passes through the diffraction slit as thescene is scanned.

In various embodiments the diffraction thickness is between 25 micronsand 100 microns.

In some embodiments the at least one companion sensor is a 4096 pixellinear panchromatic sensor array. And in some of these embodiments thepanchromatic sensor array has a five micron pixel pitch.

In other embodiments the height of the diffraction slit is 20 microns.

Another general aspect of the present invention is a method forfabricating an image panel for a hyperspectral camera that is configuredto scan a scene and obtain spectral image data over a defined range ofwavelengths. The method includes fabricating at least one companionsensor on a planar imaging surface of a silicon die, back-thinning atleast a region of the silicon die to a diffraction thickness that issuitable for a diffraction slit, and forming a diffraction slit in thethinned region, the diffraction slit penetrating the silicon die in thethinned region, the diffraction slit being co-planar with the imagingsurface of the silicon die.

In embodiments, the diffraction thickness is between 25 microns and 100microns. In certain embodiments, n forming the diffraction slit includesat least one of etching and laser-cutting the silicon die in theback-thinned region.

Yet another general aspect of the present invention is a method forfabricating an image panel for a hyperspectral camera that is configuredto scan a scene and obtain spectral image data over a defined range ofwavelengths. The method includes fabricating at least one companionsensor on a planar imaging surface of a silicon die, back-thinning atleast a region of the silicon die to a diffraction thickness that issuitable for a diffraction slit, the back-thinned region extending to anedge of the die, and forming a diffraction slit that is co-planar withthe imaging surface by using the silicon die with the thinned region asa first blade and combining it with a second blade, so that thediffraction slit is formed between a segment of the thinned region atthe edge of the silicon die and the second blade.

In embodiments, the second blade is a conventional diffraction slitblade. In certain embodiments the second blade is a second silicon diehaving a back-thinned region extending to an edge of the second silicondie. And in some of these embodiments at least one companion sensor isfabricated in the second silicon die.

The features and advantages described herein are not all-inclusive and,in particular, many additional features and advantages will be apparentto one of ordinary skill in the art in view of the drawings,specification, and claims. Moreover, it should be noted that thelanguage used in the specification has been principally selected forreadability and instructional purposes, and not to limit the scope ofthe inventive subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective functional view of the basic elements of ahyperspectral camera of the prior art;

FIG. 1B is a close-up, front view of the focused image and diffractionslit of the camera of FIG. 1A;

FIG. 2A is a front view of a diffraction slit surrounded by companionsensors in an embodiment of the present invention where the diffractionslit penetrates the silicon die;

FIG. 2B is a front view of a diffraction slit formed between two blades,each of which is a silicon die that includes companion sensors;

FIG. 2C is a front view of a diffraction slit formed between a silicondie blade that includes a companion sensor and a conventional blade;

FIG. 3A is a perspective functional diagram showing a line from a scenebeing focused on a first row of companion sensors adjacent to adiffraction slit in an embodiment of the invention;

FIG. 3B is a perspective functional diagram showing the line from thescene being focused on the diffraction slit in the embodiment of FIG.3A; and

FIG. 3C is a perspective functional diagram showing the line from thescene being focused on a second row of companion sensors adjacent to thediffraction slit in the embodiment of FIG. 3A.

DETAILED DESCRIPTION

The present invention is an apparatus for detecting push-broomhyperspectral data cube images that includes simultaneous detection oflight from the scene using companion sensors without reducing the amountof light reaching the hyperspectral sensor and without requiringseparate companion sensor optics. The apparatus takes advantage of theunused portions 118 of the scene image that are focused by the lens 100onto the image panel 104 but do not pass through the diffraction slit106. The fact that the additional data from the companion sensors isacquired through the same lens 100 and at nearly the same time and viewangle as the hyperspectral data greatly simplifies the process ofregistering the companion sensor data with the hyperspectral image,since the optics are shared and hence inherently aligned.

Simply attaching companion sensors to the image panel 104 would beproblematic, because the packaging of the companion sensors wouldconsume space and separate the companion sensors from each other andfrom the diffraction slit 106. Also, the packaging of the companionsensors would tend to locate the companion sensors above the imagingplane of the lens 100.

With reference to FIG. 2A, the apparatus of the present inventionovercomes these problems by using the silicon die 200 on which thecompanion sensors 202, 204 are fabricated as an integral part of thediffraction slit assembly, either by fabricating the diffraction slitwith one or both blades made of a companion sensor die or wafer, or, asillustrated in FIG. 2, by etching or laser cutting the diffraction slit206 directly into the silicon die 200. In embodiments, one of thetechniques known in the art for etching or laser cutting in MicroElectro-Mechanical System (MEMS) applications is used to produce a slitof a desired width.

Although typical silicon dies are too thick (300-700 μm) to serve asdiffraction slit blades, the thickness of the silicon die 200 in thepresent invention is reduced at least in a region 208 surrounding theslit 206 to a width that is suitable for the diffraction slit 206. Thisback-thinning of the silicon die is effected by any of severaltechniques known in the art, for example any of the techniques that areroutinely used to produce back-illuminated Charge Coupled Device (CCD)sensors. In embodiments, the die is thinned to a thickness of between 25μm and 100 μm, which is similar to the thickness of blades that are usedin prior art spectrometer slits. In some embodiments, the back surfaceof the die 200 is coated or aluminized to ensure that the thinnedsilicone region 208 is opaque over the full range of wavelengths to bedetected by the hyperspectral sensor array.

In embodiments, companion sensors occupy locations 202 where they willobtain data immediately after the hyperspatial sensor 110. In some ofthese embodiments, data obtained by the hyperspectral sensor 110 is usedto identify regions of interest in the scene, and to cue acquisition orspecial processing of data by the companion sensors 202. In otherembodiments, companion sensors occupy locations 204 where they willobtain data immediately before the hyperspatial sensor 110. In some ofthese embodiments, data obtained from a companion sensor 204 is used tocue actions by the hyperspatial sensor 110. For example, data storagecapacity can be conserved by using the companion sensors 204 to cue theactivation of the hyperspatial sensor 110, so that hyperspatial data isobtained only for scenes of interest.

The embodiment illustrated in FIG. 2A includes a 4096 pixel linearpanchromatic sensor array 202, 204 with 5 micron pixel pitch located oneither side of a 20 micron high diffraction slit 206 just beyond theback-thinned region 208 of the silicon wafer. The diffraction slit 206is etched or laser cut through the thinned region 208. The resolution ofthe panchromatic images will be approximately five times greater thanthe resolution of the hyperspectral data, and locating arrays 202, 204on either side of the slit 206 provides “before” 202 and “after” 204panchromatic images, which simplifies automatic alignment of thepanchromatic data with the hyperspectral data.

FIG. 2B is a top view of an image panel similar to FIG. 2A, but formedby two silicon die blades 200A and 200B, where the two blades are shownas slightly separated for clarity of illustration. Companion sensors202, 204 are fabricated into each of the die blades 200A, 200B. FIG. 2Cis a top view of an image panel similar to FIG. 2B, but formed by onesilicon die blade 200 and one conventional blade 210, where the twoblades are shown as slightly separated for clarity of illustration.Companion sensors 202 are fabricated into the die blade.

FIGS. 3A through 3C illustrate the focusing of a hypothetical “line” 300included in a scene onto the silicon die 200 of FIG. 2 as the scenemoves past the lens 100 of the hyperspectral camera (or the camera movespast the scene). In FIG. 3A, the line 300 of the scene is approachingthe center of the field of view of the lens 100, and is focused onto thecompanion sensor array 204 that is below the diffraction slit 206. InFIG. 3B, the line 300 has reached the center of the field of view of thelens 100, and is focused onto the diffraction slit 206. In FIG. 3C, theline 300 has moved past the center of the field of view of the lens 100,and is focused on the companion sensor array 202 that is above thediffraction slit 206.

Those skilled in the art will also appreciate that embodiments of thepresent invention enable calibration of the hyperspectral data to beaccomplished with reduced time and cost, improved repeatability, andwith the ability for periodic updates.

The foregoing description of the embodiments of the invention has beenpresented for the purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed. Many modifications and variations are possible in light ofthis disclosure. It is intended that the scope of the invention belimited not by this detailed description, but rather by the claimsappended hereto.

1. An image panel for use in a hyperspectral camera that is configuredto scan a scene and obtain spectral image data over a defined range ofwavelengths, the image panel comprising: a silicon die having a planarimage surface, a rear surface, and a die thickness; a thinned region ofthe silicon die having a diffraction thickness that is equal to or lessthan the die thickness; a diffraction slit formed at least in part bythe thinned region of the die, the diffraction slit having a width and aheight; and at least one companion sensor fabricated in the silicon dieand co-planar ii with the diffraction slit, the companion sensor beinglocated such that an image of a scene focused onto the diffraction slitwill also be focused onto the companion sensor.
 2. The image panel ofclaim 1, wherein the thinned region of the die extends to a side of thedie, and the diffraction slit is bounded in part by a segment of thethinned region at the side of the die.
 3. The image panel of claim 1,wherein the diffraction slit penetrates the thinned region of the die.4. The image panel of claim 1, wherein the companion sensor is apanchromatic sensor.
 5. The image panel of claim 1, wherein thecompanion sensor is configured to detect light at a wavelength that isoutside of the defined range of wavelengths.
 6. The image panel of claim1, wherein a coating is applied to at least a portion of the rearsurface, the coating rendering the portion of the rear surfacesubstantially opaque to light over the defined range of wavelengths. 7.The image panel of claim 1, wherein at least a portion of the rearsurface is metalized, the metalized portion of the die beingsubstantially opaque to light over the defined range of wavelengths. 8.The image panel of claim 1, wherein the companion sensor is located suchthat it will receive light from a portion of the scene immediatelybefore light from the portion of the scene passes through thediffraction slit as the scene is scanned.
 9. The image panel of claim 1,wherein the companion sensor is located such that it will receive lightfrom a portion of the scene immediately after light from the portion ofthe scene passes through the diffraction slit as the scene is scanned.10. The image panel of claim 1, wherein the diffraction thickness isbetween 25 microns and 100 microns.
 11. The image panel of claim 1,wherein the at least one companion sensor is a 4096 pixel linearpanchromatic sensor array.
 12. The image panel of claim 11, wherein thepanchromatic sensor array has a five micron pixel pitch.
 13. The imagepanel of claim 1, wherein the height of the diffraction slit is 20microns.
 14. A method for fabricating an image panel for a hyperspectralcamera that is configured to scan a scene and obtain spectral image dataover a defined range of wavelengths, the method comprising: fabricatingat least one companion sensor on a planar imaging surface of a silicondie; back-thinning at least a region of the silicon die to a diffractionthickness that is suitable for a diffraction slit; and forming adiffraction slit in the thinned region, the diffraction slit penetratingthe silicon die in the thinned region, the diffraction slit beingco-planar with the imaging surface of the silicon die.
 15. The method ofclaim 14, wherein the diffraction thickness is between 25 microns and100 microns.
 16. The method of claim 14, wherein forming the diffractionslit includes at least one of etching and laser-cutting the silicon diein the back-thinned region.
 17. A method for fabricating an image panelfor a hyperspectral camera that is configured to scan a scene and obtainspectral image data over a defined range of wavelengths, the methodcomprising: fabricating at least one companion sensor on a planarimaging surface of a silicon die; back-thinning at least a region of thesilicon die to a diffraction thickness that is suitable for adiffraction slit, the back-thinned region extending to an edge of thedie; and forming a diffraction slit that is co-planar with the imagingsurface by using the silicon die with the thinned region as a firstblade and combining it with a second blade, so that the diffraction slitis formed between a segment of the thinned region at the edge of thesilicon die and the second blade.
 18. The method of claim 17, whereinthe second blade is a conventional diffraction slit blade.
 19. Themethod of claim 17, wherein the second blade is a second silicon diehaving a back-thinned region extending to an edge of the second silicondie.
 20. The method of claim 19, wherein at least one companion sensoris fabricated in the second silicon die.